Locked loop circuit with reference signal provided by un-trimmed oscillator

ABSTRACT

An electronic device disclosed herein includes a locked loop circuit configured to receive a reference signal intended to have an intended frequency, wherein the locked look circuit is intended to generate an intended output signal having an intended frequency equal to the intended frequency multiplied by an intended multiplier. A frequency counter counts a number of pulses of the reference signal during a time window so as to determine an actual frequency of the reference signal. A control circuit determines an actual multiplier for the locked loop circuit that, when multiplied by the actual frequency of the reference signal, causes the locked loop circuit to generate an actual output signal having an actual frequency equal to the intended frequency.

TECHNICAL FIELD

This disclosure is directed to the field of locked loop circuits, and, in particular, to locked loop circuits capable of self-adjusting for process variations in oscillators providing reference frequencies used by those locked loop circuits.

BACKGROUND

A locked loop circuit operates to produce an output signal having a frequency that is a multiple, or fraction, of a reference frequency. In some cases, the reference frequency may be provided by a crystal oscillator. While a crystal oscillator accurately generate the reference frequency, it may be undesirably expensive, and may consume an undesirable amount of area.

Therefore, an RC oscillator may be used to generate the reference frequency for a locked loop circuit. However, due to process variations in fabrication of resistors and capacitors, an RC oscillator without a feedback mechanism may inaccurately generate such the reference frequency, which would result in the locked loop circuit employing that RC oscillator not actually generating its output signal as having a desired frequency.

Consequently, an RC oscillator used with a locked loop circuit will conventionally employ a feedback loop, calibration, or compensation circuit and be adjusted based thereupon. A digital to analog converter (DAC) is employed to generate control signals for components within the RC oscillator. By adjusting the operation of these components, the frequency of the signal generated by the RC oscillator may be adjusted.

Unfortunately, to obtain a high degree of accuracy in feedback signal generation with an RC oscillator, the DAC utilizes a large number of bits as input, resulting in the DAC being undesirably large and consuming an undesirable amount of area. In addition, a large DAC can consume an undesirable amount of power.

Therefore, further developments in the area of locked loop circuits is needed to address these issues.

SUMMARY

Disclosed herein is an electronic device including a locked loop circuit configured to receive a reference signal, and to generate therefrom an output signal, with the output signal having a frequency that is a fractional or integer multiple of a frequency of the reference signal. A frequency counter circuit is configured to measure the frequency of the reference signal and to generate a count signal based thereupon. A control circuit is configured to adjust a multiplier used by the locked loop circuit to generate the output signal based upon the count signal.

The control circuit may be configured to adjust the multiplier such that the frequency of the output signal matches a desired frequency. The frequency of the reference signal may not be equal to a desired reference frequency, and the control circuit may be configured to adjust the multiplier such that the frequency of the output signal is equal to the desired reference frequency multiplied by a desired multiplier.

The frequency counter circuit may measure the frequency of the reference signal by counting pulses of the reference signal over a time period based upon a desired accuracy of the frequency of the output signal. The time period may be equal to a number of cycles of a calibration frequency equal in number to 1 divided by a desired accuracy threshold.

An oscillator may generate the reference signal in an un-trimmed manner. The oscillator does not contain a digital to analog converter and is not a crystal oscillator.

The locked loop circuit may be a phase locked loop circuit or a frequency locked loop circuit or any other frequency multiplier circuit.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of an electronic device including a locked loop circuit in accordance with this disclosure.

FIG. 2 is a schematic block diagram of an electronic device including a digital phase locked loop circuit in accordance with this disclosure.

FIG. 3 is a schematic block diagram of an electronic device including an analog phase locked loop circuit in accordance with this disclosure.

FIG. 4 is a schematic block diagram of an electronic device including an analog frequency locked loop circuit in accordance with this disclosure.

DETAILED DESCRIPTION

One or more embodiments of the present disclosure will be described below. These described embodiments are only examples of the presently disclosed techniques. Additionally, in an effort to provide a concise description, some features of an actual implementation may not be described in the specification. When introducing elements of various embodiments of the present disclosure, the articles “a,” “an,” and “the” are intended to mean that there are one or more of the elements. The terms “comprising,” “including,” and “having” are intended to be inclusive and mean that there may be additional elements other than the listed elements.

With initial reference to FIG. 1, an electronic device 50 is now described. This electronic device 50 may be, or may be incorporated within, a mobile electronic device such as a smartphone, tablet, smartwatch, or wearable. In some cases, the electronic device 50 may form part of a single integrated circuit chip, or may form part of a single package or single module.

The electronic device 50 includes a locked loop 58, which operates to generate an output signal Fout having a frequency f_(out) that is a fractional or integer multiple of a frequency f_(ref) of a reference signal Frcosi. A RC oscillator 52 provides the reference signal Frcosi to the locked loop 58. As will be explained in greater detail below, the frequency of the reference signal Frcosi may be inaccurate in that the frequency f_(actual) Frcosi may not be the frequency f_(intended) that Frcosi is intended to have. A frequency counter 54 measures the actual frequency f_(actual) of the reference signal Frcosi in comparison to a comparison frequency f_(calibrate) of a calibration signal Fcal, and generates an output Count to control circuitry 56. The control circuitry 56 receives a desired fractional or integer multiplier Mult as an input, and adjusts the multiplier Mult_LL actually used by the locked loop 58, based upon Count, so that the frequency f_(out) of the output signal Fout is actually equal to f_(intended)*Mult.

Stated another way, the reference signal Frcosi is intended to have the frequency f_(intended), but actually has the frequency f_(actual). The desired multiplier for f_(intended) is Mult. The desired frequency f_(out) for the output signal Fout is therefore f_(intended)*Mult. However, since Frcosi actually has the frequency f_(actual) the frequency f_(out) of the output signal Fout would be f_(actual)*Mult, which is not equal to f_(intended)*Mult. Therefore, the control circuitry 56 adjusts the multiplier Mult_LL used by the locked loop 58, such that the output signal Fout has a frequency f_(out) of f_(actual)*Mult_LL, which is equal to f_(intended)*Mult.

The RC oscillator 52 generates the reference signal Frcosi in an “un-trimmed” fashion. As will be understood by those of skill in the art, “un-trimmed” means that output of the RC oscillator 52 is not adjusted for process variations. As will also be understood by those of skill in the art, a RC oscillator, such as the RC oscillator 52, requires some form of feedback, calibration, or compensation in order to precisely generate a signal having a desired frequency. Without that feedback, calibration, or compensation the frequency of the generated signal may be too high or too low, potentially by a large amount (i.e. 20%). Such mechanisms typically involve a digital to analog converter used to precisely control current sources within the RC oscillator to compensate for the fact that the actual resistance of the resistors and the actual capacitance of the capacitors is not the intended resistance and the intended capacitance and thereby ensure that the generated signal has the desired frequency.

Is therefore made explicitly clear here that the RC oscillator 52 uses no feedback, calibration, or compensation mechanisms, uses no digital to analog converter, and generates its output signal Frcosi having an actual frequency f_(actual) that is not equal to the desired frequency f_(intended), and is instead higher or lower than f_(intended). There are no mechanisms to correct Frcosi itself, and Frcosi remains uncorrected. Thus, it is also made explicitly clear that the locked loop 58 may not receive a reference signal Frcosi having the intended frequency f_(intended), but instead may receive the reference signal Frcosi having the incorrect and undesired frequency f_(actual) as the RC oscillator 52 is free running and un-trimmed.

The frequency counter 54 determines the actual frequency f_(actual) of the reference signal Frcosi by counting the number of pulses in Frcosi over a time window measured in clock cycles of Fcal and related to a desired accuracy of the frequency. The desired accuracy can be represented as a percentage, and in that case, the time window has a duration of 100/DesiredAccuracy clock cycles, referred to as Count_Ref. For example, if the desired accuracy is 0.1%, then the window has a duration of 100/0.1=1000 cycles at a frequency of Fintended. Therefore, if the frequency counter 54 counts 800 cycles (where the desired accuracy is 0.1%), it is known that f_(actual) is 20% less than f_(desired), within an accuracy of 0.1%.

The output Count generated by the frequency counter 54 is thus a digital representation of: Count=Count_Ref*f_(actual)/f_(intended).

The control circuitry 56 receives Count, and then calculates the multiple Mult_LL to be used by the locked loop 58 as: MMult_LL=(Count_Ref/Count)*Mult=f _(intended)*Mult/f _(actual).

Therefore, the frequency of Fout will be f_(out)=f_(actual)*Mult_LL=f_(intended)*Mult. Thus, the electronic device 50 has realized an accurate locked loop circuit 58 that receives its reference frequency from an un-trimmed RC oscillator 52. By using the un-trimmed RC oscillator 52, the amount of surface area consumed by the RC oscillator 52 is greatly reduced, as the digital to analog converters utilized in trimmed RC oscillators can consume as much, or more than, 70% of the available surface area. In addition, power consumption is reduced due to the lack of trimming.

In addition, since there is no surface area penalty for increasing the accuracy of the output signal Fout from the locked loop 58, the frequency of the reference signal Frcosi can be greatly reduced and Mult_LL can be increased accordingly to produce the desired output frequency f_(out). This helps to further reduce power consumption. Moreover, the effect of temperature on the reference signal Frcosi is reduced by the reduction in frequency, as is the effect of transient switching.

It should be understood that the locked loop 58 which may be a frequency locked loop or a phase locked loop, and may be constructed from entirely analog components or a combination of analog or digital components. In the case where the locked loop 58 is a frequency locked loop, the locked loop 58 operates to generate an output signal Fout that has a frequency f_(out) that is a desired multiple Mult of a desired frequency f_(intended) of a reference signal Frcosi, and in the case where the locked loop 58 is a phase locked loop, in addition to the output signal Fout having a frequency f_(out) that is the desired multiple Mult of the desired frequency f_(intended) of the reference signal Frcosi, the phase of the output signal Fout is locked to the phase of the reference signal Frcosi.

An embodiment in which the locked loop 58 is a digital phase locked loop is now described with reference to FIG. 2. Operation of the RC oscillator 52, frequency counter 54, and control circuitry 56 remains the same as above and will not be repeated for brevity. Here, the locked loop 58 includes a phase frequency detector (PFD) 22. The PFD 22 receives as input the reference signal Frcosi and a feedback signal Fmult. The PFD 22 determines a difference in phase between the reference frequency signal Frcosi and the feedback signal Fmult and outputs a digital signal Ddif indicative of that measured difference. The digital signal Ddif is filtered by digital filter 26, such as a low pass digital filter, which generates a control signal Dcont. A digital-to-analog converter (DAC) 28 converts the digital control signal Dcont to an analog control signal Acont. A control input of an oscillator circuit 30, such as a voltage controlled oscillator or current controlled oscillator, receives the analog control signal Acont and generates an output clock signal Fout having a frequency f_(out) that is dependent on the magnitude of the analog control signal Acont. A divider circuit (/N) 32 divides the output clock signal Fout by N (equal to Mult_LL) to generate a feedback signal Fmult.

An embodiment in which the locked loop 58′ is an analog phase locked loop is now described with reference to FIG. 3. Operation of the RC oscillator 52, frequency counter 54, and control circuitry 56 remains the same as above and will not be repeated for brevity. Here, the locked loop 58′ includes a phase frequency detector (PFD) 22. The PFD 22 receives as input the reference signal Frcosi and a feedback signal Fmult. The PFD 22 determines a difference in phase between the reference frequency signal Frcosi and the feedback signal Fmult and outputs an analog signal Adif indicative of that measured difference. The analog signal Adif is filtered by filter 26′, such as a low pass analog filter, which generates a control signal Acont. A control input of an oscillator circuit 30, such as a voltage controlled oscillator or current controlled oscillator, receives the analog control signal Acont and generates an output clock signal Fout having a frequency f_(out) that is dependent on the magnitude of the analog control signal Acont. A divider circuit (/N) 32 divides the output clock signal Fosc by N (equal to Mult_LL) to generate a feedback signal Fmult.

An embodiment in which the locked loop 58″ is an analog frequency locked loop is now described with reference to FIG. 3. Operation of the RC oscillator 52, frequency counter 54, and control circuitry 56 remains the same as above and will not be repeated for brevity. Here, the locked loop 58″ includes a frequency comparator 22. The frequency comparator 22 receives as input the reference signal Frcosi and the feedback signal Fmult. The frequency comparator 22 determines a difference in frequency between the reference frequency signal Frcosi and the feedback signal Fmult and outputs an analog signal Fdif indicative of that measured difference. The analog signal Fdif is filtered by filter 26″, such as a low pass analog filter, which generates a control signal Cont. A control input of an oscillator circuit 30, such as a voltage controlled oscillator or current controlled oscillator, receives the control signal Cont and generates an output clock signal Fout having a frequency f_(out) that is dependent on the magnitude of the control signal Cont. A divider circuit (/N) 32 divides the output clock signal Fout by N (equal to Mult_LL) to generate a feedback signal Fmult.

While the disclosure has been described with respect to a limited number of embodiments, those skilled in the art, having benefit of this disclosure, will appreciate that other embodiments can be envisioned that do not depart from the scope of the disclosure as disclosed herein. Accordingly, the scope of the disclosure shall be limited only by the attached claims. 

The invention claimed is:
 1. An electronic device, comprising: an RC oscillator configured to generate a reference signal; a locked loop circuit configured to receive the reference signal, and to generate therefrom an output signal, the output signal having a frequency that is a multiple of a frequency of the reference signal, wherein the multiple is set by a division value applied by a frequency divider in a feedback path of the locked loop circuit; wherein the locked loop circuit comprises: a frequency detector configured to generate a comparison signal as a function of a comparison between the reference signal and a feedback signal; and an oscillator configured to generate the output signal as a function of the comparison signal; wherein the frequency divider is configured to divide the output signal by the division value to produce the feedback signal as having the frequency that is the multiple of the frequency of the reference signal; a frequency counter circuit configured to measure the frequency of the reference signal and to generate a count signal based thereupon, wherein the frequency counter circuit is not coupled to the feedback signal and is configured to receive the reference signal and a comparison signal, and to measure the frequency of the reference signal by comparing it to the comparison signal; and a control circuit configured to adjust the division value used by the locked loop circuit to generate the output signal based upon the count signal.
 2. The electronic device of claim 1, wherein the control circuit is configured to adjust the division value such that the frequency of the output signal matches a desired frequency.
 3. The electronic device of claim 1, wherein the frequency of the reference signal is not equal to a desired reference frequency; and wherein the control circuit is configured to adjust the division value such that the frequency of the output signal is equal to the desired reference frequency multiplied by a desired multiplier.
 4. The electronic device of claim 1, wherein the frequency counter circuit measures the frequency of the reference signal by counting pulses of the reference signal over a time period based upon a desired accuracy of the frequency of the output signal.
 5. The electronic device of claim 4, wherein the time period is equal to a number of cycles of a calibration frequency equal in number to 1 divided by a desired accuracy threshold.
 6. The electronic device of claim 1, wherein the RC oscillator generates the reference signal in an un-trimmed manner.
 7. The electronic device of claim 6, wherein the RC oscillator does not contain a digital to analog converter.
 8. The electronic device of claim 1, wherein the locked loop circuit is a phase locked loop circuit.
 9. The electronic device of claim 1, wherein the locked loop circuit is a frequency locked loop circuit.
 10. The electronic device of claim 1, wherein the comparison signal is not the feedback signal.
 11. The electronic device of claim 1, wherein the locked loop circuit is further configured to receive a feedback signal and to generate the output signal as a function of a comparison between the reference signal and the feedback signal; and wherein the frequency counter circuit is configured to measure the frequency of the reference signal by comparing it to a comparison signal.
 12. The electronic device of claim 1, wherein the frequency of the reference signal is not equal to a desired reference frequency; wherein the multiple is not equal to a desired multiple; and wherein the control circuit adjusts the division value such that the frequency of the output signal is equal to the desired reference frequency multiplied by the desired multiple.
 13. The electronic device of claim 1, wherein the control circuit receives a non-zero intended division value, and adjusts the division value by adjusting the received non-zero intended division value based upon the count signal.
 14. A method, comprising: generating a reference signal using an RC oscillator; receiving a feedback signal; operating a locked loop circuit to generate an output signal from the reference signal, the output signal having a frequency that is a multiple of a frequency of the reference signal, wherein the multiple is set by a division value applied by a frequency divider in a feedback path of the locked loop circuit, wherein the locked loop circuit is operated to generate the output signal from the reference signal as a function of a comparison between the reference signal and the feedback signal; measuring the frequency of the reference signal and generating a count signal based thereupon, wherein the frequency of the reference signal is measured by comparing it to a comparison signal, the comparison signal being separate and distinct from the feedback signal; and adjusting the division value used by the locked loop circuit to generate the output signal, based upon the count signal.
 15. The method of claim 14 wherein the division value is adjusted such that the frequency of the output signal matches a desired frequency.
 16. The method of claim 14, wherein the frequency of the reference signal is not equal to a desired reference frequency; and wherein the division value is adjusted such that the frequency of the output signal is equal to the desired reference frequency multiplied by a desired multiplier.
 17. The method of claim 14, wherein the frequency of the reference signal is measured by counting pulses of the reference signal over a time period based upon a desired accuracy threshold of the frequency of the output signal.
 18. The method of claim 17, wherein the time period is equal to a number of cycles of a calibration frequency equal in number to 1 divided by the desired accuracy threshold.
 19. The method of claim 14, wherein the frequency of the reference signal is not equal to a desired reference frequency; wherein the multiple is not equal to a desired multiple; and wherein the division value is adjusted such that the frequency of the output signal is equal to the desired reference frequency multiplied by the desired multiple.
 20. The method of claim 14, further comprising receiving a non-zero intended division value; and wherein the division value is adjusted by adjusting the received non-zero intended division value. 